Bioassays that detect and quantify biomolecules at ultra-low concentrations are of great need in many fields, including basic medical science, disease control and diagnostics, drug discovery, and environment monitoring. Bioassays can be used for disease or pathogen detection based on specific interactions between oligonucleotides, such as DNA-DNA or RNA-RNA interactions; small-molecule-biomolecule interactions; aptamer-biomolecule interactions; protein interactions; or the like.
Magnetic biosensing techniques utilize magnetic fields to detect and quantify biomolecules. In some implementations, magnetic labels, such as magnetic nanoparticles (MNPs) or magnetic microbeads, can be attached to an analyte in a reagent. The same analyte, which is not attached to a magnetic label, may be present in a serum sample. The reagent and serum sample may be introduced into a sensor that includes a magnetic sensor and a plurality of capture molecules, which are configured to capture the analyte, e.g., using covalent or ionic bonding. The magnetic sensor may include a magnetic layer whose magnetic moment is fixed in a particular orientation (a fixed layer) and a magnetic layer whose magnetic moment is free to rotate under influence of an external magnetic field (a free layer). When an external applied field is applied to the sensor, e.g., using an electromagnet or a permanent magnet, the magnetic moment of the free layer rotates to an orientation determined by the effective magnetic field applied to the layer, which may include components from the external applied field, magnetic fields from any magnetic objects (e.g., magnetic particles above the sensor's surface, the magnetic field generated by a current that passes through the magnetic sensor during a read process, and the magnetic field generated by other magnetic layers (e.g., the fixed layer) of the magnetic sensor. The concentration of the analyte in the serum sample then may be determined using the magnetic biosensor, as the orientation of the magnetic moment of the free layer will rotate under influence of the magnetic markers when analytes bonded to magnetic markers are captured by the capture molecules. Further details of magnetic biosensors and related techniques are described in U.S. Provisional Application No. 61/534,636, incorporated herein by reference.
In some magnetic biosensors, an in-plane external magnetic field configuration is used (i.e., the external magnetic field is applied to the sensor in a direction parallel to a major plane of the free layer and fixed layer). The instrumentation and working principle for this configuration is illustrated schematically in FIG. 1. In this configuration, the sample plate 18 including one or more samples 10 is placed on one or more magnetic sensors (not shown) between two opposing magnets 12 and 14 that generate an in-plane magnetic field represented by arrows 16 (i.e., in plane relative to a major plane of the free magnetic layer). This in-plane configuration can be utilized for a limited size of testing plate (e.g., 2-5 centimeters (cm) in length), but cannot be used with large sample plates (e.g., 50 cm in length) unless large magnets 12 and 14 are used, which is not compatible with a bench top system. Large magnets are required because a uniform, relatively large (e.g., about 20 Oerstad (Oe) to about 100 Oe) in-plane magnetic field is required for accurate detection of biomolecules in the samples being tested. This configuration with large electromagnet is not only unsuitable for bench top operation but also requires large amounts of power to operate the large electromagnet.